This invention relates to a process for the isolation of an aromatic hydroxycarboxylic acid from its mono- or dialkali metal salts. More particularly, this invention relates to such a process in which these salts are electrodialyzed in the presence of other selected alkali metal salts to reduce overvoltage near the end of the electrolysis. Alkali metals and their hydroxides may be completely and economically recycled in the process.
Aromatic hydroxycarboxylic acids and dicarboxylic acids are important components in the manufacture of commercial products. For example, p-hydroxybenzoic acid (PHBA) is used to make parabens and is also used as a monomer in making polyesters, and salicyclic acid (o-hydroxybenzoic acid) is used to make aspirin. Traditionally, aromatic hydroxycarboxylic acids are manufactured using the Kolbe-Schmitt reaction, which is a reaction of an alkali metal salt of an aromatic hydroxy compound with carbon dioxide, usually under elevated temperature and pressure. The Kolbe-Schmitt reaction has been a standard procedure for the preparation of aromatic hydroxy acids for over 100 years; see for instance A. S. Lindsey, et al., Chem. Rev., vol. 57, p. 583-620 (1957) incorporated by reference herein. However, this process is complex and difficult to run, involving several manufacturing steps, which adds to the cost of the final product. Since the initial product of the carboxylation reaction is a dialkali metal salt of the aromatic hydroxycarboxylic acid, substantial cost is usually incurred for the use of compounds such as NaOH or KOH. These compounds are subsequently discarded (as sodium or potassium salts), since the free aromatic hydroxycarboxylic acid is usually isolated by reacting the dialkali metal salt with a strong acid. The dialkali metal salt of a hydroxycarboxylic acid may also be completely electrodialyzed to the free aromatic hydroxycarboxylic acid, but when one tries to completely electrodialyze these compounds (and as one approaches complete electrolysis), the voltage increases and the current efficiency decreases rapidly. As a result, the process may become impractical and/or uneconomical. Japanese Patent Application 40-11492 describes the electrodialysis of an alkali metal salt of terephthaiic acid to terephthalic acid and an alkali metal hydroxide. Japanese Patent Application 64-9954 describes the electrodialysis of an alkali metal salt of hydroxybenzoic acid.
None of the above references describes an electrodialysis in which an alkali metal salt is added to prevent overvoltage. Therefore, it would be desirable to improve these electrodialysis processes by reducing or eliminating the overvoltage without adversely affecting their advantages.
There is disclosed and claimed herein a process for the preparation of an aromatic hydroxycarboxvlic acid from its dialkali metal salt, comprising electrodialyzing an aqueous solution of a first compound of the formula (OR1CO2)HtM2xe2x88x92t, and a second compound of the formula MxHzQ, to produce a third compound of the formula (OR1CO2)HyM2xe2x88x92y and MOH, wherein:
R1 is arylene;
t is zero to about 1.90;
each M is independently an alkali metal cation;
HzQ is an anion whose conjugate acid has a pKa of about 2 or less;
x is an integer of 1 or more, and z is 0 or an integer of 1 or more, provided that x+z is equal to the total number of negative charges on Q;
M has a concentration associated with HzQ at a pH of 2.5 of about 0.03 to about 4 molar; and
y is about 1.95 to 2.00.